The major barriers to preventing or treating Alzheimer's disease (AD) are its unknown pathogenesis/etiology and the lack of a sufficiently sensitive and specific objective biomarker of the disease, particularly at the early stages when therapeutic interventions would likely have the greatest efficacy. The basic hypothesis of this proposal is that levels of a novel protein-protein complex present in cerebrospinal fluid (CSF) are able to differentiate AD subjects from age-matched controls and subjects with non-AD neurologic disorders. In addition, the biomarker complex appears to identify subjects with mild cognitive impairment (MCI), the earliest clinical manifestation of AD. Analysis of lumbar CSF from a small number of living probable AD and age-matched controls showed 100% sensitivity and 93% specificity in the identification of AD subjects. The proposed studies will enhance our understanding of the way in which the protein complex is generated and will further validate our initial findings by: 1) quantifying levels of the protein-protein complex in small volumes of ventricular CSF from autopsy-verified subjects with non-AD pathologies, MCI, LAD, and age-matched control subjects using an enzyme linked immunoassay developed in our laboratory based on the trapping one component of the complex and quantifying the other component;2) quantifying levels of the protein-protein complex in lumbar CSF from living probable AD and age-matched control subjects using the ELISA and 3) using sequential immunoprecipitation, SDS-PAGE and mass spectrometry to examine individual components of the protein complex in ventricular CSF from MCI, LAD and age-matched control subjects. Together these studies will provide further validation of our preliminary findings and will provide support for wide scale testing of the protein-protein complex as a biomarker of AD and MCI. PUBLIC HEALTH RELEVANCE: Alzheimer's disease (AD) is the fourth leading cause of death in the United States and currently affects 4.5 million Americans. Two major barriers to treating and eventually preventing AD are: 1) the lack of understanding of the process of neuron degeneration and loss and 2) the lack of a sufficiently sensitive and specific biomarker of the disease. Preliminary and future studies described in this proposal show that a novel protein-protein complex present in cerebrospinal fluid can be quantified using an enzyme linked immunoassay (ELISA) and that this protein-protein complex is a sensitive and specific biomarker of AD including early stages of AD (mild cognitive impairment) when therapeutic interventions are most likely to have beneficial effects.